Rotary mill

ABSTRACT

A rotary mill is disclosed comprising a body portion ( 4 ) having a milling surface ( 10 ) and a central bore ( 20 ) extending along an axis of rotation ( 18 ) of the body portion ( 4 ); and a guide portion ( 6 ) having a guide body ( 22 ) and a guide peg ( 24 ) extending from the guide body ( 22 ), the guide peg ( 24 ) operable to be received in the central bore ( 20 ) of the body portion ( 4 ), the guide body ( 22 ) having at least one alignment feature ( 28, 30 ) which is the same as that of a prosthesis component. A method of implanting a unicondylar femoral component is also disclosed, the method comprising, a) reaming the femoral condylar surface to accept the unicondylar femoral component; b) drilling peg holes for affixing the unicondylar femoral component; c) affixing a guide portion of a rotary mill onto the prepared condylar surface using the drilled peg holes; d) reaming a portion of bone anterior to the affixed guide portion; e) removing the guide portion from the bone; and f) affixing a unicondylar femoral component to the prepared condylar surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 U. S. National Stage of InternationalApplication No. PCT/GB2012/050159, filed on Jan. 26, 2012 and publishedas WO 2012/101441 A1 on Aug. 2, 2012. This application claims priorityto British Patent Application No. 1101377.8, filed on Jan. 27, 2011. Thedisclosures of the above applications are incorporated herein byreference in their entirety.

The present invention relates to rotary mills and similar rotary cuttingdevices and particularly but not exclusively relates to rotary mills foruse in preparing a bone for total or partial joint replacement surgery.

BACKGROUND

It is known to replace all or part of a knee joint in which the jointsurfaces have deteriorated, for example as a result of osteoarthritis.Such deterioration usually starts in only one of the tibeo-femoralcompartments and may spread to the other at a later stage. Replacementof only one compartment of the joint can therefore be sufficient toprovide prolonged relief from symptoms. Damaged bearing surfaces arereplaced by a unicompartmental prosthesis which comprises a femoralimplant and a tibial implant (usually metallic), which interface througha (polyethylene) bearing component disposed between the two implants.

A unicompartmental or partial knee replacement (PKR) helps to conserveundamaged bone and restores more natural movement to the joint. Also,owing to the small size of the prosthesis, the surgery may be lessinvasive than a total knee replacement (TKR). However, the designrequirements for partial knee replacement prostheses are more demandingthan those for total knee replacement prostheses. Unlike in a total kneereplacement, where one or more ligaments can be discarded and themechanics of the knee can be simplified, in a unicompartmental kneereplacement, all the ligaments in the joint must be retained andrestored to their natural tensions and the bearing component must becompletely unconstrained.

During articulation of the knee, and particularly when the joint is atfull extension, the bearing component can impinge on femoral condylarbone tissue superior to the femoral implant, as illustrated in FIG. 1.Such impingement of the polyethylene bearing component onto the bone canlead to post operative pain, damage to the bearing, increased wear andeventual failure. It is therefore essential to remove a sufficientamount of anterior bone on the femoral condyle during the implantationprocedure to prevent such impingement from occurring.

Orthopaedic surgeons conventionally use a bone chisel to manually removethe anterior bone. However, such a manual procedure can easily beforgotten during surgery and, even when carefully completed, results inan undesirable non uniform bone edge and in the removal of an uncertainand varying amount of bone.

SUMMARY OF INVENTION

According to the present invention, there is provided a rotary millcomprising a body portion having a milling surface and a central boreextending along the rotary axis of the body portion; and a guide portionhaving a guide body and a guide peg extending from the guide body, theguide peg operable to be received in the central bore of the bodyportion, the guide body having at least one alignment feature which isthe same as that of a prosthesis component.

By forming the guide body to have at least one alignment feature whichis the same as that of a prosthesis component, the guide portion can beapplied to the bone and fixed in place using the pre prepared fixationfeatures that will hold the prosthesis component in place (for examplepeg holes or a recess for a flange). The guide body thus provides aprecise reference of the final location of the prosthesis component tobe implanted. The guide peg thus guides the body portion to mill an areaof bone that is in a precise and predetermined location with respect tothe eventually implanted prosthesis component.

A further advantage to the guide body being formed in this manner isthat no additional bone must be removed in order for it to be fixed onthe bone surface. The guide portion, having at least one alignmentfeature which is the same as that of a prosthesis component, can fitinto the necessary recesses already formed in the bone to accommodatethe final prosthesis component. This is in contrast to conventionalguided mills which require a dedicated hole to be drilled to accommodatea separate guide rod.

The at least one alignment feature may be an attachment peg or may be apair of attachment pegs. The attachment pegs may be operable to bereceived in pre prepared prosthesis peg holes.

The guide body may have substantially the form of a trial prosthesiscomponent and may in fact comprise a trial prosthesis component.

The guide body may include at least one nodule, protruding from asurface of the guide body and operable to abut a corresponding abutmentsurface on the body portion. The nodules may thus act as depth stops toensure a precise amount of bone is removed and avoid excessive boneremoval.

The guide peg may comprise an abutment surface operable to abut acorresponding abutment surface in the central bore of the body portion.The guide peg may thus not only act to guide the angle at which the bodyportion mills bone surface but may also act as a depth stop to limitbone removal.

The abutment surface may comprise a distal surface of the peg or theabutment surface may comprise an outwardly projecting annular shoulder.

The corresponding abutment surface of the central bore may comprise abase of the bore or may comprise an inwardly projecting annularshoulder.

The guide peg may project from a predetermined region of, and at apredetermined angle to the guide body. In this manner the region of boneto be removed may be precisely determined and fixed by the constructionof the guide portion, facilitating accuracy and repeatability ofmilling.

The guide peg may be adjustable on the guide body, allowing the surgeona degree of freedom in selection of the milled area, and to accommodatefor different patient geometries.

The guide body may be operable to be connected to additional surgicaltools, thus facilitating and providing reference for additional boneremoval steps.

The guide body may comprise a trial femoral prosthesis component whichmay be a trial unicondylar femoral prosthesis component.

The guide peg may project from an anterior portion of the guide body andmay be operable to guide reaming of a region superior to the anterioredge of the guide body.

The guide peg may project from the guide body at an angle of between 25and 40 degrees to the axis of the attachment peg. The angle may varyaccording to the size of the rotary mill.

The guide body may be operable to be connected to a posterior osteophyteguide.

The rotary mill may further comprise additional guide portions, eachguide portion being of a different size so as to match differently sizedprosthesis components that are employed for patents of differing sizes.

The rotary mill may further comprise additional guide portions, eachguide portion having a different length, and or angle of extension ofguide peg, thus also accommodating different patent geometries.

The body portion of the rotary mill may comprise a rotary body and aguide shaft at least partially received within the rotary body.

The guide shaft may comprise an inner portion telescopically receivedwithin an outer portion and a biasing element acting between the innerand outer portions.

The guide shaft may be received within an axial bore which may be formedin the rotary body. The bore may be a blind bore.

The biasing element may comprise a spring which may be mounted about theinner portion of the guide shaft.

The body portion may further comprise cooperating protrusions formed onthe rotary body and the outer portion of the guide shaft, operable toengage one another as a depth stop.

The cooperating protrusions may comprise annular shoulders which may beformed on an inner surface of the rotary body and an outer surface ofthe outer portion of the guide shaft.

The outer portion of the guide shaft may comprise a substantially hollowshaft, a distal end of which may comprise the central bore of the bodyportion, operable to receive the guide peg.

The cutting surface of the body portion may be formed on an annularcutting tool which may be removably attached to the rotary body.

According to another aspect of the present invention, there is provideda method of implanting a unicondylar femoral component comprising,

-   -   a) reaming the femoral condylar surface to accept the        unicondylar femoral component;    -   b) drilling peg holes for affixing the unicondylar femoral        component;    -   c) affixing a guide portion of a rotary mill onto the prepared        condylar surface using the drilled peg holes;    -   d) reaming a portion of bone anterior to the affixed guide        portion;    -   e) removing the guide portion from the bone; and    -   f) affixing a unicondylar femoral component to the prepared        condylar surface.

The rotary mill may be a rotary mill according to the first aspect ofthe present invention.

According to another aspect of the present invention, there is provideda rotary cutting tool comprising a rotary body having a cutting surfaceand a guide shaft at least partially received within the rotary body,wherein the guide shaft comprises an inner portion telescopicallyreceived within an outer portion, and a biasing element acting betweenthe inner and outer portions.

The biasing element of the tool thus acts to damp the telescoping motionof the guide shaft and hence, when received within the rotary body,damps progression of the rotary body along the guide shaft. This dampingaction can assist a surgeon with fine control of cutting or millingoperations.

The guide shaft may be received within an axial bore formed on therotary body. The bore may be a blind bore.

The rotary cutting tool may further comprise cooperating protrusionsformed on the rotary body and the outer portion of the guide shaft,which protrusions may be operable to engage one another as a depth stop.

The protrusions may for example comprise annular shoulders which may beformed on an inside surface of the rotary body and an outer surface ofthe outer portion of the guide shaft.

The biasing element may comprise a spring. The spring may be mountedabout the inner portion of the guide shaft. The spring may act betweenan end of the outer portion of the guide shaft and an end cap formed onthe inner portion of the guide shaft. The end cap may be formed by anend of the axial bore in which the guide shaft is received.

The outer portion of the guide shaft may comprise a hollow shaft, afirst end of which may receive the inner portion and a second end ofwhich may be operable to receive a guide peg.

The cutting surface of the rotary cutting tool may be formed on anannular cutting plate which may be removably attached to the rotarybody.

An end of the rotary body may terminate in an annular receiving plate,which may be operable to engage the cutting plate.

The rotary cutting tool may further comprise cooperating formations onthe receiving plate and cutting plate, which may be operable to securethe cutting plate to the receiving plate.

The cooperating formations may comprise protrusions, for example screwheads, and appropriately shaped recesses. The formations mayalternatively or further comprise magnetic elements.

Another end of the rotary body may terminate in a drive shaft which maybe operable to engage a drive element such as a rotary drill.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the present invention, and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the following drawings, in which:

FIG. 1 shows impingement of a meniscal bearing on anterior femoral bone.

FIG. 2 is a perspective view of a rotary mill.

FIG. 3 is a perspective view of a body portion of a rotary mill.

FIG. 4 shows a guide portion of a rotary mill in position on a femur.

FIG. 5 shows a rotary mill in position on a femur.

FIG. 6 illustrates bone removal by a rotary mill.

FIGS. 7 and 8 illustrate prosthesis components in position on a femur,with and without bone removal.

FIG. 9 illustrates an implanted unicondylar prosthesis.

FIG. 10 is a sectional view of a rotary cutting tool.

FIG. 11 is a sectional view of the tool of FIG. 10 in a compressedcondition.

DETAILED DESCRIPTION

With reference to FIGS. 2 to 4, a rotary mill 2 comprises a body portion4 and a guide portion 6. The body portion 4 comprises a rotary body 8that terminates at one end in an annular milling surface 10. The millingsurface comprises a series of milling teeth 12 that extend from thesurface 10. In the illustrated embodiment, the milling surface 10 isformed on an annular shoulder 14 that protrudes outwardly from therotary body 8. An abutment surface 15 extends radially inwardly of theprojecting milling surface 10. At a second end of the rotary body 8 anintegral shank 16 extends along an axis of rotation 18 of the rotarybody. The shank 16 is suitable for attachment to the chuck of a surgicaldrill (not shown). A bore 20 extends through the rotary body 8 along theaxis of rotation 18 of the rotary body 8.

The guide portion 6 comprises a guide body 22 and a guide peg 24. Theguide body 22 comprises a trail unicondylar femoral prosthesiscomponent. The guide body thus comprises a curved condylar plate 26 andtwo attachment pegs 28, 30. The attachment pegs 28, 30 protrude from abone contacting surface 32 of the condylar plate 26 at locations andangles precisely matching those of a similarly sized prosthesiscomponent. The guide body can thus be attached to a prepared femoralcondylar surface in the same manner and using the same drilled peg holesas for a prosthesis component. In this manner, the attachment pegs 28,30 serve to align the guide body with the eventual location of thefemoral prosthesis, referencing off the pre prepared drilled femoral pegholes. The guide peg 24 is a cylindrical peg that protrudes from anopposite, outer surface 34 of the condylar plate 26. The guide peg 24extends from an anterior portion of the condylar plate 26 along an axisthat is substantially normal to the adjacent condylar plate surface 34.The guide peg 24 projects at an angle of between 25 and 40 degrees tothe axis of the attachment pegs. The precise angle is selected accordingto the size of the guide portion and associated anatomy, as discussed infurther detail below. The guide peg 24 is dimensioned to be slidably androtatably received within the central bore 20 of the rotary body 8. Withreference also to FIG. 6, two nodules 27, 29 protrude from the outersurface 34 of the condylar plate 26. The nodules 27, 29 are formed onopposite sides of the anterior portion of the condylar plate 26, in theregion of the guide peg 24.

The rotary mill 2 is used to remove anterior bone on the femoral condyleprior to implantation of a unicondylar femoral prosthesis. First, thecondylar surface is prepared to receive the prosthesis, includingresection of the entire condylar surface and drilling of femoral pegholes. The guide portion 6 of the rotary mill 2 is then fixed on thefemoral condyle by inserting the attachment pegs 28, 30 into the predrilled femoral peg holes. The guide portion 6 can be seen in positionon the femoral condyle in FIG. 4. The body portion 4 is then seated onthe guide portion 4, the guide peg 24 being receiving within the bore 20of the rotary body 8. The shank 16 of the body portion is then attachedto a surgical drill and the body portion 4 is guided to mill the bonesuperior to the anterior edge of the condylar plate 26 of the guideportion. The guide peg 24 guides the orientation of the body portion 4,ensuring that the milling surface 10 removes the bone tissue from thecorrect location. The body portion 4 advances along the guide peg 24 asbone tissue is removed. As the body portion 4 advances, the nodules 27,29 act as stop pegs, upper surfaces of the nodules 27, 29 abutting thestop surface 15 that extends radially inwardly from the annular millingsurface 10 and preventing further movement of the body portion 4, thuslimiting the amount of bone that is removed. The body portion 4 isdimensioned so as to ream only the anterior bone superior to the guideportion. As can be seen from FIGS. 5 and 6, the milling surface 10 doesnot disturb the adjacent soft tissues and so causes minimal damage ordisruption to the surrounding structures, removing only that bone whichis desired to be removed. As illustrated particularly in FIG. 5, theguide peg 24 is angled such that, when fully seated on the guide peg 24,the body portion 4 has only reamed the bone superior to the anterioredge of the guide portion 6. The reamed area of bone can be seen at area38 on FIG. 6.

Additional tools can then be attached to the guide portion if necessary.For example, a posterior osteophyte guide 40 can be attached posteriorlyto the guide portion 6. The osteophyte guide is a slotted tool that maybe used to guide a chisel to remove osteophytes from the posterior areaof the femoral condyle, helping to prevent femoral loosening.

Once all necessary bone removal has been completed, the guide portion 6of the rotary mill 2 is removed and the appropriate prosthesis componentis implanted. FIGS. 7 and 8 illustrate the area 38 of bone that isremoved by the rotary mill 2. On FIG. 8, this area 38 can be seenimmediately superior to the femoral prosthesis component 50. On FIG. 7,the rotary mill 2 has not been used and bone tissue remains superior tothe prosthesis component 50. This bone tissue will cause impingement ofthe meniscal component, as illustrated in FIG. 1. In contrast, and asillustrated in FIG. 9, when the mill has been used to remove bone overthe desired area 38, no impingement of the meniscal component is seen,even with the knee in full extension.

It is envisaged that the guide peg 24 may be integral with the guidebody 22 or may be detachable from, or adjustable relative to, the guidebody 22, so as to allow limited adjustment of the angle of the guide peg24 or of the height of the guide peg 24. Such adjustment allows a degreeof flexibility to the surgeon in tailoring the rotary mill 2 to theprecise needs of individual patients. For example, if it is desired toremove less than the usual amount of bone, the guide peg 24 may becaused to protrude further from the surface of the condylar plate 26. Inthis instance, the guide peg 24 also acts as a stop peg, the end surface36 of the guide peg 24 contacting the base (not shown) of the bore 20and preventing further movement. The guide peg may be caused to protrudeto such an extent that it is engages as a stop peg before the stopsurface 15 of the body portion 4 contacts the nodules 27, 29 of theguide body. It is also envisaged that the guide portion 6 of the rotarymill be provided as merely one of several available guide portions, eachbeing of a different size to accommodate different sizes of knee. Thus,each size of prosthesis may have an associated guide portion 6 of theappropriate size. Each guide portion 6 will have a suitable guide peg,of a height and at an angle that is determined to be most appropriatefor the associated prosthesis.

It will be appreciated that the guide portion 6 may be employed togetherwith other embodiments of body portion 4, including a range of rotarycutting devices. One embodiment of rotary cutting tool with which theguide portion 6 may be employed is illustrated in FIGS. 10 and 11. Therotary cutting tool 100 comprises a rotary body 102 and a guide shaft104. The guide shaft 104 is at least partially received within a blindaxial bore 106 formed within the rotary body 102. A closed proximal end(towards the left in the Figures) of the rotary body 102 terminates in adrive shaft 108, operable to be received within the chuck of a surgicaldrill (not shown). An open distal end of the rotary body 102 flaresoutwards to terminate in an annular receiving plate 110 extending aboutthe opening of the axial bore 106 and described in further detail below.

The guide shaft 104 comprises an inner portion 112 and an outer portion114. The inner portion 112 comprises a solid shaft a distal end 116 ofwhich is telescopically received within a proximal end 118 of the outerportion 114. The outer portion 114 comprises a substantially hollowshaft. A biasing spring 120 is mounted about the inner portion 112 ofthe guide shaft 104. The spring 120 rests at one end on the annular endsurface 122 of the proximal end 118 of the outer portion 114. The otherend of the spring 120 engages on an end cap 124 formed on a proximal end126 of the inner portion 112. In an alternative embodiment (not shown)the spring 120 may engage on the blind end 128 of the axial bore 106 inwhich the guide shaft 104 is received.

The guide shaft is received freely within the bore 106 of the rotarybody 102. An annular shoulder 130 is formed on the inner surface of thebore 106, dividing the bore into a distal section and a proximalsection, the distal section being of larger inside diameter than theproximal section. A corresponding annular shoulder 132 is formed on theouter surface of the outer portion 114 of the guide shaft, dividing theouter portion into proximal and distal sections, the distal sectionbeing of larger outside diameter than the proximal section. Thecorresponding annular shoulders 130, 132 function as a depth stop,preventing the outer portion 114 of the guide shaft 104 from beingreceived into the rotary body 102 beyond a certain point. This positionis illustrated in FIG. 11. The larger diameter distal section of theouter portion 114 of the guide shaft may also serve to centre the guideshaft within the bore 106 of the rotary body 102.

Referring particularly to FIG. 10, the annular receiving plate 110comprises a series of formations, operable to releasably engage anannular cutting plate (not shown). The formations comprise at least onescrew head 134 and a plurality of magnets 136, the magnets beingrecessed into the annular receiving plate so as to present a smoothsurface. The annular cutting plate (not shown) comprises an annularcutting surface, similar to that described above with reference to thebody portion 6, and an opposed annular engaging surface. The annularengaging surface comprises corresponding recesses and magnetic elementsenabling the cutting plate to be releasably yet securely attached to thereceiving plate 110 of the rotary body.

In use, the rotary cutting tool 100 is first assembled and then placedover the guide peg 24 of the guide portion 6. The guide peg 24 isreceived within the hollow outer portion 114 of the guide shaft until adistal end 138 of the outer portion 114 is seated against the surface 34from which the guide peg 24 protrudes. The rotary body 102 is thenconnected to a surgical drill (not shown) via the drive shaft 108 andthe rotary tool is guided to mill away the desired area of bone. Duringthe cutting operation, the outer portion 114 of the guide shaft 104remains seated in position over the guide peg 24. Downward pressureapplied to the rotary body engages the blind end 128 of the bore 106against the end cap of the inner portion 112 of the guide shaft 104,causing the inner portion 112 to be pushed further into the outerportion 114. This action compresses the spring 120 acting between theinner and outer portions. In this manner, the spring 120 damps thedownward motion of the rotary body, assisting the control of the surgeonand thus increasing the ease with which the tool may be employed. Theinner portion 112 of the guide shaft 104 continues to slide further intothe outer portion 114 until the annular shoulder 130 on the rotary body102 engages the annular shoulder 132 on the outer portion 114 of theguide shaft. At this point the rotary body cannot travel any furthertowards the bone and the drilling action is ceased. In this manner, theannular shoulders act as a depth stop, preventing over reaming of thebone.

It will be appreciated that the present invention provides a means foraccurately, predictably and repeatably removing a targeted area of bonefrom the femoral condyle. The amount of bone removed is determined bythe precise angle and height of the guide peg 24. These aspects of theguide peg 24 are determined when the guide portion 6 is initially formedand can thus be carefully assessed and fixed so as to guide milling ofprecisely the correct amount of bone for the associated prosthesis. Thepresent invention is also bone conserving, requiring no additional drillhole for a guide rod, as is conventionally required for a guided mill.By fastening to the bone using the existing femoral peg holes, the guideportion 6 makes use of existing features, and requires no additionalbone removal for fixation. The femoral peg holes in fact determine theeventual location of the milled bone, as they provide the location forthe guide portion 6. As these peg holes also provide location for thefinal prosthesis component, considerable time and development effort hasbeen devoted to tools and techniques to ensure the accurate positioningof the peg holes in the femur. The present invention makes indirect useof these pre-existing tools and techniques in employing the femoral pegholes as the fixation means for the guide portion 6 of the rotary mill2.

The present invention additionally provides a rotary cutting tooloptimised for use with the guide portion 6, the action of which isdamped or cushioned, improving ease of use for the surgeon.

The invention claimed is:
 1. A rotary mill comprising: a body portionhaving a milling surface and a central bore extending along an axis ofrotation of the body portion; and a guide portion having a guide bodyforming a condylar plate and a guide peg extending from the condylarplate and operable to be received in the central bore of the bodyportion, the guide body having at least one alignment feature which isthe same as that of a prosthesis component, wherein the guide bodyincludes at least one nodule, protruding from a surface of the condylarplate and operable to abut a corresponding abutment surface on the bodyportion, the at least one nodule protruding outwardly from the condylarplate.
 2. The rotary mill as claimed in claim 1, wherein the at leastone alignment feature is an attachment peg.
 3. The rotary mill asclaimed in claim 2, wherein the guide peg projects from the condylarplate at an angle of approximately between 25 and 40 degrees to an axisof the attachment peg.
 4. The rotary mill as claimed in claim 1, whereinthe at least one alignment feature is a pair of attachment pegs.
 5. Therotary mill as claimed in claim 1, wherein the guide peg comprises anabutment surface operable to abut a corresponding abutment surface inthe central bore of the body portion.
 6. The rotary mill as claimed inclaim 5, wherein the abutment surface comprises a distal surface of thepeg.
 7. The rotary mill as claimed in claim 5, wherein the abutmentsurface comprises an outwardly projecting annular shoulder.
 8. Therotary mill as claimed in claim 7, wherein the corresponding abutmentsurface of the central bore comprises an inwardly projecting annularshoulder.
 9. The rotary mill as claimed in claim 1, wherein the guidepeg projects from a predetermined region of and at a predetermined angleto the guide body.
 10. The rotary mill as claimed in claim 1, whereinthe guide peg is adjustable on the guide body.
 11. The rotary mill asclaimed in claim 1, wherein the guide body is operable to be connectedto additional surgical tools.
 12. The rotary mill as claimed in claim 1,wherein the condylar plate forms a trial femoral prosthesis component.13. The rotary mill as claimed in claim 12, wherein the guide body isoperable to be connected to a posterior osteophyte guide.
 14. The rotarymill as claimed in claim 1 wherein the guide body comprises a trialunicondylar femoral prosthesis component.
 15. The rotary mill as claimedin claim 14, wherein the guide peg projects from an anterior portion ofthe guide body.
 16. The rotary mill as claimed in claim 15, wherein theguide peg is operable to guide reaming of a region superior to theanterior portion of the guide body.
 17. The rotary mill as claimed inclaim 1, further comprising additional guide portions, each guideportion being of a different size.
 18. The rotary mill as claimed inclaim 1, further comprising additional guide portions, each guideportion having a different length or angle of extension of guide peg.19. A rotary cutting tool comprising; a rotary body having a cuttingsurface formed on an annular cutting plate removably attachable to therotary body, wherein an end of the rotary body terminates in an annularreceiving plate having cooperating formations comprising a fastener anda plurality of magnets operable to engage the annular cutting plate andsecure the annular cutting plate to the receiving plate; and a guideshaft at least partially received within the rotary body, wherein theguide shaft comprises an inner portion telescopically received within anouter portion; and a biasing element acting between the inner and outerportions.
 20. The rotary cutting tool as claimed in claim 19, whereinthe guide shaft is received within an axial bore formed on the rotarybody.
 21. The rotary cutting tool as claimed in claim 19, furthercomprising cooperating protrusions formed on the rotary body and theouter portion of the guide shaft, operable to engage one another as adepth stop.
 22. The rotary cutting tool as claimed in claim 19, whereinthe biasing element comprises a spring.
 23. The rotary cutting tool asclaimed in claim 22, wherein the spring is mounted about the innerportion of the guide shaft.
 24. The rotary cutting tool as claimed inclaim 22, wherein the spring acts between an end of the outer portion ofthe guide shaft and an end cap formed on the inner portion of the guideshaft.
 25. The rotary cutting tool as claimed in claim 24, wherein theend cap is formed by an end of an axial bore in which the guide shaft isreceived.
 26. The rotary cutting tool as claimed in claim 19, whereinthe outer portion of the guide shaft comprises a hollow shaft, a firstend of which receives the inner portion and a second end of which isoperable to receive a guide peg.
 27. A rotary mill comprising: a bodyportion having a milling surface and a central bore extending along anaxis of rotation of the body portion; and a guide portion having a guidebody and a guide peg extending from the guide body, the guide pegoperable to be received in the central bore of the body portion, theguide body having at least one alignment feature which is the same asthat of a prosthesis component, wherein the guide body comprises a trialunicondylar femoral prosthesis component and the guide peg projects froman anterior portion of the guide body, the guide peg operable to guidereaming of a region superior to the anterior portion of the guide body.28. The rotary mill as claimed in claim 27, wherein the body portioncomprises a rotary body and a guide shaft at least partially receivedwithin the rotary body and the guide shaft comprises an inner portiontelescopically received within an outer portion and a biasing elementacting between the inner and outer portions.
 29. The rotary mill asclaimed in claim 28, wherein the biasing element comprises a springmounted about the inner portion of the guide shaft.
 30. The rotary millas claimed in claim 28, further comprising cooperating protrusionsformed on the rotary body and the outer portion of the guide shaft,operable to engage one another as a depth stop.
 31. The rotary mill asclaimed in claim 28, wherein the outer portion of the guide shaftcomprises a substantially hollow shaft, a distal end of which comprisesthe central bore of the body portion.
 32. A rotary mill comprising: abody portion including a milling surface, a central bore extending alongan axis of rotation of the body portion, a rotary body, and a guideshaft at least partially received within the rotary body; and a guideportion having a guide body forming a condylar plate and a guide pegextending from the condylar plate and operable to be received in thecentral bore of the body portion, the guide body having at least onealignment feature which is the same as that of a prosthesis component.33. The rotary mill as claimed in claim 32, wherein the guide shaftcomprises an inner portion telescopically received within an outerportion and a biasing element acting between the inner and outerportions.
 34. The rotary mill as claimed in claim 33, wherein thebiasing element comprises a spring mounted about the inner portion ofthe guide shaft.
 35. The rotary mill as claimed in claim 33, furthercomprising cooperating protrusions formed on the rotary body and theouter portion of the guide shaft, operable to engage one another as adepth stop.
 36. The rotary mill as claimed in claim 33, wherein theouter portion of the guide shaft comprises a substantially hollow shaft,a distal end of which comprises the central bore of the body portion.37. The rotary mill as claimed in claim 32, wherein the milling surfaceis formed on an annular cutting tool removably attached to the rotarybody.